In the aspirating particle detection systems, such as the Vesda® range of smoke detectors manufactured by Xtralis Pty Ltd, a network of sampling pipes is routed over an area to be monitored by the particle detection system. FIG. 1 illustrates such a particle detection system 10. The system 10 includes the particle detector 12 coupled to a sample pipe network 14 comprised of two sampling pipes 16. Although, the network may include further sampling pipes, or conversely, a single sampling pipe. Each sampling pipe includes a plurality of air sampling points 18. The air sampling points 18 may be a simple hole in the sampling pipe 16 or a fitting that couples to the pipe 16 and has a hollow generally cylindrical frusto-conical body with a hole at the end, into which air is drawn. Such a fitting can be directly connected to the air sampling pipes, e.g. by being interposed in the pipe or attached to a T-junction directly, or connected thereto by a length of hose. In use, air is drawn into the air sample inlets 18 and into the particle detector 12 by an aspirator 20. The aspirator 20 typically forms part of the particle detector 12. Air drawn through the system 10 enters the detector at detector inlet 21 and passes, through inlet plenum 26 having a flow sensor 24, to the aspirator 20. The aspirator then outputs the air to an outlet plenum 28, from where most of the air is exhausted back to the atmosphere via an exhaust port 22. The air pressure in the outlet plenum 28 is at high pressure, compared to the air at the inlet plenum 26, so a portion of the air is fed back to the inlet plenum via (optional) dust filter 30 and particle detection chamber 32, where any particles of interest are detected.
The aspirator 20 delivers sample air drawn from the ambient air in the volume (e.g. room or cabinet etc) that is being monitored to the detector 12 at a predetermined flow rate. As will be appreciated the flow rate will vary depending on system parameters, but will typically be in the range of 10 to 150 liters per minute. FIG. 1A is a graph showing flow rate 13 delivered to the particle detector 12 vs time. During operation of the particle detection system 10, the flow rate 13 in each sampling pipe 16 and/or to the detector 12 is set by configuring the aspirator 20 to run nominally at the predetermined flow rate. However, the flow rate 13 may vary from the nominal flow rate 15a due to external environmental influences or blockages at one or more sampling point. Therefore, the flow rate is monitored by flow sensor 24 to ensure that the flow rate is within a specified allowable range. The allowable range is typically set by upper 15b and lower 15c flow thresholds, which are typically percentage deviations from the nominal flow 15a rate. Short transient variations in flow (having a duration less than a predetermined delay period) outside the specified range are not necessarily indicative of a problem in the sampling network and may therefore be ignored. However, if the flow rate is outside the specified range for longer than a fault delay period 17, it will be determined that the predetermined flow rate is not being met. The function or reliability of particle detector may be compromised, and a fault signal issued.
The inventors have determined that, some users of aspirating particle detection systems may wish to monitor an outside area, rather than an inside area for which such systems are generally intended. However, outside environments present a challenge in that an ambient flow of air about the sampling points, for example due to wind, can result in ambient air being pushed into, or drawn from, the sampling points, and associated sampling network, causing an increase or decrease in flow beyond the acceptable limits. Due to the localised nature of turbulence and flow speeds of ambient relative air flow between different sampling points may also become unbalanced. As illustrated in FIG. 1A, if the flow rate 13 increases beyond the acceptable range for a period 19a, less than the fault delay period 17, the predetermined flow rate is considered to be met, so no fault signal is issued. However, some time later, the flow rate 13 decreases beyond the acceptable range for a period 19b, which is longer than the fault delay period 17, the predetermined flow rate is considered to not be met, so a fault signal is issued.
The present invention addresses the above challenge in using an aspirated particle detection system in an outdoor environment.
Reference to any prior art in the specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in Australia or any other jurisdiction or that this prior art could reasonably be expected to be ascertained, understood and regarded as relevant by a person skilled in the art.